Optical interrogator for performing interferometry using fiber Bragg gratings

ABSTRACT

There is described a method for interrogating optical fiber comprising fiber Bragg gratings (“FBGs”), using an optical fiber interrogator. The method comprises (a) generating an initial light pulse from phase coherent light emitted from a light source, wherein the initial light pulse is generated by modulating the intensity of the light; (b) splitting the initial light pulse into a pair of light pulses; (c) causing one of the light pulses to be delayed relative to the other of the light pulses; (d) transmitting the light pulses along the optical fiber; (e) receiving reflections of the light pulses off the FBGs; and (f) determining whether an optical path length between the FBGs has changed from an interference pattern resulting from the reflections of the light pulses.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of and priority to Canada PatentApplication No. 2,970,205, filed on Jun. 8, 2017, which is incorporatedherein by reference.

TECHNICAL FIELD

The present disclosure is directed at an optical interrogator forperforming interferometry using fiber Bragg gratings.

BACKGROUND

Optical interferometry is a technique in which two separate lightpulses, a sensing pulse and a reference pulse, are generated andinterfere with each other. When optical interferometry is used for fiberoptic sensing applications, the sensing and reference pulses are atleast partially reflected back towards an optical receiver. For example,optical interferometry may be performed by directing the sensing andreference pulses along an optical fiber that comprises fiber Bragggratings, which partially reflect the pulses back towards an opticalreceiver at which interference is observed. The nature of theinterference observed at the optical receiver provides information onthe optical path length the pulses traveled, which in turn providesinformation on parameters such as the strain the optical fiberexperienced.

The circuitry that generates, modulates, and receives the sensing andreference pulses is typically contained within a device called anoptical interrogator. There exists a continued desire to advance andimprove technology used in optical interrogators.

SUMMARY

In a first aspect of the disclosure, there is provided an optical fiberinterrogator for interrogating optical fiber comprising fiber Bragggratings (“FBGs”), the interrogator comprising: (a) a light sourceoperable to emit phase coherent light; (b) amplitude modulationcircuitry optically coupled to the light source and operable to generateone or more light pulses from the light; (c) an optical splitteroptically coupled to the amplitude modulation circuitry and beingconfigured to split a light pulse received from the amplitude modulationcircuitry into a pair of light pulses; (d) an optical delayer opticallycoupled to the optical splitter and configured to introduce a delay toone light pulse of the pair of light pulses relative to the other lightpulse of the pair of light pulses; and (e) control circuitry comprisinga controller, communicatively coupled to the amplitude modulationcircuitry, and configured to perform a method for interrogating theoptical fiber comprising generating a light pulse by using the amplitudemodulation circuitry to modulate light emitted by the light source,wherein the generated light pulse is split into a pair of light pulses(“first and second light pulses”) by the optical splitter, and whereinone of the light pulses is delayed relative to the other light pulse bythe optical delayer.

The optical delayer may comprise a delay coil or a delay-on-chipcircuit, such as one or more of those described athttp://www.darpa.mil/news-events/2013-11-26.

The interrogator may further comprise a phase modulator opticallycoupled to the amplitude modulation circuitry and operable to introducea phase shift to at least one of the first and second light pulses. Themethod may further comprise phase shifting one of the first and secondlight pulses relative to the other of the first and second light pulsesby using the phase modulator.

The phase modulator may further comprise a solid state phase modulator.The phase modulator may be selected from the group consisting of alithium niobate phase modulator, a gallium arsenide phase modulator, andan indium phosphide phase modulator.

The interrogator may further comprise (a) an output optical amplifieroptically coupled to the phase modulator; (b) receiver circuitry; and(c) an optical circulator comprising first, second, and third ports,wherein the first port is optically coupled to the output opticalamplifier, the second port is optically coupled to an output of theinterrogator for respectively sending and receiving the pair of lightpulses to and from the optical fiber, and the third port is opticallycoupled to the receiver circuitry for processing signals received fromthe optical fiber.

The interrogator may further comprise polarization maintaining fiberbetween the light source and the phase modulator such that apolarization of the light emitted by the light source is maintained fromthe light source to the phase modulator.

The interrogator may further comprise single mode fiber between thephase modulator and the output of the interrogator.

The interrogator may further comprise single mode fiber between theoutput of the interrogator and the receiver circuitry.

The interrogator may further comprise an optical combiner opticallycoupled to the optical delayer and the optical splitter. The opticalcombiner may be configured to receive the pair of lights pulses viarespective inputs of the optical combiner, and transmit the pair oflight pulses via a common output of the optical combiner.

The interrogator may further comprise an optical attenuator opticallycoupled between the third port of the optical circulator and thereceiver circuitry, for attenuating an intensity of light input to theoptical attenuator.

The interrogator may further comprise a polarization splitter opticallycoupled between the third port of the optical circulator and thereceiver circuitry, for splitting light input to the polarizationsplitter as a function of a polarization of the incident light.

The interrogator may further comprise (a) receiver circuitry; and (b) anoptical circulator comprising first, second, and third ports, whereinthe first port is optically coupled to the phase modulator, the secondport is optically coupled to an output of the interrogator forrespectively sending and receiving the pair of light pulses to and fromthe optical fiber, and the third port is optically coupled to thereceiver circuitry for processing signals received from the opticalfiber.

The light source may comprise a laser having a power of at least 100 mW.

The phase shifting may comprise applying a positive phase shift to oneof the light pulses (“the first light pulse”), and applying a negativephase shift to the other light pulse, intended to interfere with thefirst light pulse. The phase shifting may comprise phase shifting thesensing pulse by 2π and not phase shifting the reference pulse.

The first and second light pulses may differ in phase from each other bymore than π radians.

The method may further comprise: (a) generating a calibration pulse; (b)determining when reflections of the calibration pulse off the FBGsarrive at the receiver circuitry; and (c) based on differences in whenthe reflections of the calibration pulse arrive at the receivercircuitry, configuring the delay introduced by the optical delayer.

The phase shifting may comprise applying a non-linear phase shift or apiecewise linear phase shift to at least one of the first and secondlight pulses.

The phase shift may comprise a Barker code.

The method may further comprise dithering leakage from the amplitudemodulation circuitry by phase shifting the leakage between 0 and πradians at a frequency at least 2.5 times higher than a frequency atwhich interrogation is being performed.

The amplitude modulation circuitry may comprises (a) an input opticalisolator and an output optical isolator isolating an input and output ofthe amplitude modulation circuitry, respectively; and (b) an inputoptical amplifier optically coupled between the input optical isolatorand the output optical isolator.

The light source may comprise an electroabsorption modulated laser andthe amplitude modulation circuitry comprises an absorption region of theelectroabsorption modulated laser.

A first group of the FBGs may be tuned to reflect a wavelength of lightdifferent to a wavelength to which are tuned a second group of the FBGs,and wherein the light source is operable to emit multiple wavelengths oflight for interrogating the first and second groups of the FBGS. Thecontrol circuitry may be further configured to perform the method forinterrogating the optical fiber by using wavelength divisionmultiplexing.

The controller may be configured to determine phase data frominterference of reflections of the first light pulse off the FBGs withreflections of the second light pulse off the FBGs.

The interrogator may be configured to interrogate multiple opticalfibers, the interrogator further comprising an outgoing optical switchoptically coupled to the light source and comprising at least two switchoutputs, the outgoing optical switch being operable to switchtransmission of light between each of the at least two switch outputs,and wherein the control circuitry is further communicatively coupled tothe outgoing optical switch and configured to perform the method forinterrogating each of the multiple optical fibers, comprising:generating a light pulse by using the amplitude modulation circuitry tomodulate light emitted by the light source, wherein the generated lightpulse is split into a pair of light pulses (“first and second lightpulses”) by the optical splitter, and wherein one of the light pulses isdelayed relative to the other light pulse by the optical delayer; andcontrolling the outgoing optical switch to switch transmission betweenthe at least two switch outputs.

The interrogator may further comprise an incoming optical switchoptically coupled to the receiver circuitry and comprising at least twoswitch inputs, the incoming optical switch being operable to switchtransmission of light between each of the at least two switch inputs.

The control circuitry may be further communicatively coupled to theincoming optical switch and further configured to perform the method forinterrogating each of the multiple optical fibers, comprisingcontrolling the incoming optical switch to switch transmission betweenthe at least two switch inputs.

The control circuitry may be further configured to perform the methodfor interrogating each of the multiple optical fibers, comprisingcontrolling the outgoing optical switch to switch transmission betweenthe at least two switch outputs, and controlling the incoming opticalswitch to switch transmission between the at least two switch inputs,such that the multiple optical fibers are interrogated at controllableduty cycles.

The interrogator may further comprise an accelerometer for obtainingacceleration data related to vibrations of the interrogator, wherein thecontroller is communicatively coupled to the accelerometer andconfigured to carry out a method comprising: receiving the accelerationdata; determining a correlation between the acceleration data and thephase data; and adjusting the phase data as a function of thecorrelation so as to compensate for the vibrations.

The vibrations of the interrogator may comprise vibrations of one ormore of: the delay coil, the light source, and the phase modulator.

The interrogator may further comprise comprising a temperature sensorfor obtaining temperature data related to a temperature of theinterrogator, wherein the controller is communicatively coupled to thetemperature sensor and configured to carry out a method comprising:receiving the temperature data; determining a correlation between thetemperature data and the phase data; and adjusting the phase data as afunction of the correlation so as to compensate for the temperature.

The temperature of the interrogator may comprise a temperature of one ormore of: the delay coil, the light source, and the phase modulator.

The interrogator may further comprise a GPS receiver, wherein thecontroller is configured to synchronize interrogation of the opticalfiber as a function of a signal received from the GPS receiver, oranother external synchronization signal.

The controller may be further configured to determine Lissajous datafrom interference of reflections of the first light pulse off the FBGswith reflections of the second light pulse off the FBGs.

The controller may be further configured to determine Lissajous datafrom the interference of reflections of the first light pulse off theFBGs with reflections of the second light pulse off the FBGs, duringinterrogation of the optical fiber.

The controller may be configured to assemble the phase data into datapackets, each data packet comprising a key, a frame identifier and apayload comprising at least a portion of the phase data.

The controller may be configured to determine whether any of the datapackets meet a data error condition and, if so, add an indication to thedata packet that the data packet contains erroneous data.

The data error condition may be determined to be met if: the frameidentifiers of consecutively assembled data packets do not meet apredetermined requirement; or the keys of consecutively assembled datapackets do not meet a predetermined requirement.

The predetermined requirement may comprise the frame number of anearlier assembled data packet being one less than the frame number ofthe later, consecutively assembled data packet.

The predetermined requirement may comprise the key of one of theconsecutively assembled data packets being separated from the key of theother of the consecutively assembled data packets by a preset number ofbits.

The interrogator may be configured to transmit the data packets to asignal processing device communicatively coupled to the interrogator.

The interrogator and the computing device may be configured tocommunicate over a communication line with a throughput of at least 1Gb/s.

In a further aspect of the disclosure, there is provided a system forinterrogating optical fiber comprising fiber Bragg gratings (“FBGs”),the system comprising: (a) an optical fiber interrogator according toany of the above-described embodiments; and (b) one or more opticalfiber segments optically coupled to the interrogator.

The system may further comprise an outgoing optical splitter and anincoming optical combiner, the outgoing optical splitter being opticallycoupled to the light source and being configured to split light receivedat the outgoing optical splitter and transmit the split light out eachof multiple outputs of the outgoing optical splitter, and wherein theincoming optical combiner is optically coupled to the receiver circuitryand is configured to combine light received at each of multiple inputsof the incoming optical combiner and transmit the combined light to thereceiver circuitry.

The system may further comprise one or more filter and balance unitsoptically coupled to one or more of the multiple inputs of the incomingoptical combiner.

The system may further comprise one or more optical circulatorsoptically coupled to each of the one or more optical fiber segments,wherein, for each optical fiber segment, light sent from theinterrogator to the optical fiber segment passes through the opticalcirculator, is reflected off the FBGs comprised in the optical fibersegment, and is redirected by the circulator to the receiver circuitry.

The system may further comprise one or more lead-in optical fibersegments optically coupling the interrogator to each of the one or moreoptical circulators, and one or more return optical fiber segmentsoptically coupling each of the one or more optical circulators to thereceiver circuitry.

The one or more lead-in optical fiber segments may be optically coupledto the multiple outputs of the outgoing optical splitter.

The one or more return optical fiber segments may be optically coupledto the multiple inputs of the incoming optical combiner.

The one or more return optical fiber segments may be optically coupledto the one or more filter and balance units.

In embodiments, the one or more lead-in optical fiber segments and theone or more return optical fiber segments do not comprise FBGs.

The interrogator may be communicatively coupled to a signal processingdevice configured to receive the data packets from the interrogator.

The signal processing device may be further configured to determinewhether any of the data packets meet a data error condition and, if so,add an indication to the data packet that the data packet containserroneous data.

The data error condition may be determined to be met if: the frameidentifiers of consecutively assembled data packets do not meet apredetermined requirement; or the keys of consecutively assembled datapackets do not meet a predetermined requirement.

The predetermined requirement may comprise the frame number of anearlier assembled data packet being one less than the frame number ofthe later, consecutively assembled data packet.

The predetermined requirement may comprise the key of one of theconsecutively assembled data packets being separated from the key of theother of the consecutively assembled data packets by a preset number ofbits.

The signal processing device may be configured to extract the phase datafrom the data packet if no data error condition is met.

In a further aspect of the disclosure, there is provided a method forinterrogating optical fiber comprising fiber Bragg gratings (“FBGs”),using an optical fiber interrogator, the method comprising: (a)generating an initial light pulse from phase coherent light emitted froma light source, wherein the initial light pulse is generated bymodulating the intensity of the light; (b) splitting the initial lightpulse into a pair of light pulses; (c) causing one of the light pulsesto be delayed relative to the other of the light pulses; (d)transmitting the light pulses along the optical fiber; (e) receivingreflections of the light pulses off the FBGs; and (f) determiningwhether an optical path length between the FBGs has changed from aninterference pattern resulting from the reflections of the light pulses.

Determining whether the optical path length has changed may compriseconverting the interference pattern from an optical to an electricalsignal.

The method may further comprise phase shifting at least one of the lightpulses relative to the other of the light pulses.

The phase shifting may be carried out using a phase modulator. The phasemodulator may be a solid state phase modulator.

The phase modulator may be selected from the group consisting of alithium niobate phase modulator, a gallium arsenide phase modulator, andan indium phosphide phase modulator.

A polarization of the light pulses may be maintained from when theinitial light pulse is generated until the at least one of the lightpulses is phase shifted.

Single mode fiber may be used to optically couple the phase modulatorand an output of the interrogator.

The method may further comprise splitting the reflected light pulses asa function of a polarization of the reflected light pulses, prior toconverting the interference patterns.

The light source may be a laser and an intensity of the light may bemodulated using an input optical amplifier external of and opticallycoupled to the laser.

The light may be generated by an electroabsorption modulated laser andthe intensity of the light may be modulated using an absorption regioncomprising part of the laser.

The light source may comprise a laser having a power of at least 100 mW.

The phase shifting may comprise applying a positive phase shift to oneof the light pulses (“the first light pulse”), and applying a negativephase shift to the other of the light pulses (“the second light pulse”),so that the first light pulse may interfere with the second light pulse.

The first and second light pulses may differ in phase from each other bymore than π radians.

The method may further comprise (a) transmitting a calibration pulse tothe FBGs; (b) receiving reflections of the calibration pulse off theFBGs; and (c) based on differences in when the reflections of thecalibration pulse are received, determining, or configuring, the delaybetween the pair of light pulses.

The phase shifting may comprise applying a nonlinear phase shift or apiecewise linear phase shift to at least one of the light pulses. Thephase shift may comprise a Barker code.

The method may further comprise dithering leakage from the light sourceby phase shifting the leakage between 0 and π radians at a frequency atleast 2.5 times higher than a frequency at which interrogation is beingperformed.

The interrogator may configured to interrogate multiple optical fibers,the interrogator further comprising an outgoing optical switch opticallycoupled to the light source and comprising at least two switch outputs,and The method may further comprise, for each optical fiber: generatingan initial light pulse from phase coherent light emitted from the lightsource, wherein the light pulse is generated by modulating the intensityof the light; splitting the initial light pulse into a pair of lightpulses; causing one of the light pulses to be delayed relative to theother of the light pulses; and controlling the outgoing optical switchto switch transmission between the at least two switch outputs.

The interrogator may further comprise an incoming optical switchoptically coupled to receiver circuitry and comprising at least twoswitch inputs, the incoming optical switch being operable to switchtransmission of light between each of the at least two switch inputs.

The method may further comprise controlling the incoming optical switchto switch transmission between the at least two switch inputs.

The method may further comprise controlling the outgoing optical switchto switch transmission between the at least two switch outputs, andcontrolling the incoming optical switch to switch transmission betweenthe at least two switch inputs, such that the multiple optical fibersare interrogated at controllable duty cycles.

A first group of the FBGs may be tuned to reflect a wavelength of lightdifferent to a wavelength to which are tuned a second group of the FBGs,and wherein the light source may be operable to emit multiplewavelengths of light for interrogating the first and second groups ofthe FBGS, and the method may further comprise using wavelength divisionmultiplexing to distinguish the reflections of the light pulses off theFGBs.

An outgoing optical splitter may be optically coupled to the lightsource and may be configured to split light received at the outgoingoptical splitter and transmit the split light out each of multipleoutputs of the outgoing optical splitter, wherein an incoming opticalcombiner may be optically coupled to receiver circuitry and may beconfigured to combine light received at each of multiple inputs of theincoming optical combiner and transmit the combined light to thereceiver circuitry.

The method may further comprise determining phase data from interferenceof reflections of the first light pulse off the FBGs with reflections ofthe second light pulse off the FBGs.

The method may further comprise: receiving acceleration data related tovibrations of the interrogator; determining a correlation between theacceleration data and the phase data; and adjusting the phase data as afunction of the correlation so as to compensate for the vibrations.

The vibrations of the interrogator may comprise vibrations of one ormore of: the delay coil, the light source, and the phase modulator.

The method may further comprise receiving temperature data related to atemperature of the interrogator; determining a correlation between thetemperature data and the phase data; and adjusting the phase data as afunction of the correlation so as to compensate for the temperature.

The temperature of the interrogator may comprise a temperature of one ormore of: the delay coil, the light source, and the phase modulator.

The method may further comprise synchronizing interrogation of theoptical fiber as a function of a signal received from a GPS receiver.

The method may further comprise determining Lissajous data frominterference of reflections of the first light pulse off the FBGs withreflections of the second light pulse off the FBGs.

The method may further comprise determining Lissajous data from theinterference of reflections of the first light pulse off the FBGs withreflections of the second light pulse off the FBGs, during interrogationof the optical fiber.

The method may further comprise assembling the phase data into datapackets, each data packet comprising a key, a frame identifier and apayload comprising at least a portion of the phase data.

The method may further comprise determining whether any of the datapackets meet a data error condition and, if so, adding an indication tothe data packet that the data packet contains erroneous data.

Determining the data error condition may comprise: determining whetherthe frame identifiers of consecutively assembled data packets do notmeet a predetermined requirement; or determining whether the keys ofconsecutively assembled data packets do not meet a predeterminedrequirement.

The predetermined requirement may comprise the frame number of anearlier assembled data packet being one less than the frame number ofthe later, consecutively assembled data packet.

The predetermined requirement may comprise the key of one of theconsecutively assembled data packets being separated from the key of theother of the consecutively assembled data packets by a preset number ofbits.

The method may further comprise, if an erroneous condition is not met,extracting the phase data from the data packet and store the extractedphase data.

The method may further comprise transmitting the data packets to asignal processing device separate from the interrogator.

The method may further comprise using the signal processing device toextract the phase data if no data error condition is met.

The interrogator and the signal processing device may be configured tocommunicate over a communication line with a throughput of at least 1Gb/s.

In a further aspect of the disclosure, there is provided anon-transitory computer readable medium having stored thereon programcode to cause a processor to perform a method for interrogating opticalfiber comprising fiber Bragg gratings (“FBGs”), using an optical fiberinterrogator, according to any of the above-described embodiments.

This summary does not necessarily describe the entire scope of allaspects.

Other aspects, features and advantages will be apparent to those ofordinary skill in the art upon review of the following description ofspecific embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, which illustrate one or more exampleembodiments:

FIG. 1A is a block diagram of a system for detecting dynamic strain,which includes an optical fiber with fiber Bragg gratings (“FBGs”) forreflecting a light pulse, according to one embodiment.

FIG. 1B is a schematic that depicts how the FBGs reflect a light pulse.

FIG. 10 is a schematic that depicts how a light pulse interacts withimpurities in an optical fiber that results in scattered laser light dueto Rayleigh scattering, which is used for distributed acoustic sensing(“DAS”).

FIG. 2 is a schematic of an optical interrogator for performinginterferometry using FBGs, according to one embodiment.

FIG. 3 is a schematic of an optical interrogator for performinginterferometry using FBGs, according to one embodiment.

FIG. 4 is a graph of example pulses resulting from reflections ofsensing and reference pulses off of the FBGs.

FIG. 5 is a schematic of an optical interrogator for performinginterferometry using FBGs, according to one embodiment.

FIG. 6 depicts an example of pulse timing.

FIG. 7 is a method for interrogating optical fiber that comprises FBGs,according to one embodiment.

FIGS. 8A and 8B are a schematic of multiple optical interrogators forinterrogating multiple sensor loads, according to one embodiment.

FIG. 9 is a schematic of multiple optical fiber segments forming thesensor loads of FIG. 8, according to one embodiment.

FIG. 10 is a schematic of a data packet, according to one embodiment.

FIG. 11 is a method of packaging and transferring phase data from theinterrogator to a signal processing device, according to one embodiment.

FIGS. 12A and 12B show a data flow process for the interrogator,according to one embodiment.

FIGS. 12C and 12D show a method of determining edge positions, accordingto one embodiment.

FIG. 13 shows a method of calibrating the interrogator, according to oneembodiment.

DETAILED DESCRIPTION

Directional terms such as “top”, “bottom”, “upwards”, “downwards”,“vertically”, and “laterally” are used in the following description forthe purpose of providing relative reference only, and are not intendedto suggest any limitations on how any article is to be positioned duringuse, or to be mounted in an assembly or relative to an environment.Additionally, the term “couple” and variants of it such as “coupled”,“couples”, and “coupling” as used in this description are intended toinclude indirect and direct connections unless otherwise indicated. Forexample, if a first device is coupled to a second device, that couplingmay be through a direct connection or through an indirect connection viaother devices and connections. Similarly, if the first device iscommunicatively coupled to the second device, communication may bethrough a direct connection or through an indirect connection via otherdevices and connections.

Optical interferometry is a technique in which two separate light pulsesare generated: a sensing pulse and a reference pulse. These pulses maybe generated by an optical source such as a laser. When opticalinterferometry is used for fiber optic sensing applications, the sensingand reference pulses are at least partially reflected back towards anoptical receiver. Optical interferometry has a variety of applications,one of which is being used to detect dynamic strain.

Referring now to FIG. 1A, there is shown one embodiment of a system 100for performing interferometry using fiber Bragg gratings (“FBGs”). Thesystem 100 comprises optical fiber 112, an interrogator 106 opticallycoupled to the optical fiber 112, and a signal processing device 118that is communicative with the interrogator 106.

The optical fiber 112 comprises one or more fiber optic strands, each ofwhich is made from quartz glass (amorphous SiO₂). The fiber opticstrands are doped with various elements and compounds (includinggermanium, erbium oxides, and others) to alter their refractive indices,although in alternative embodiments the fiber optic strands may not bedoped. Single mode and multimode optical strands of fiber arecommercially available from, for example, Corning® Optical Fiber.Example optical fibers include ClearCurve™ fibers (bend insensitive),SMF28 series single mode fibers such as SMF-28 ULL fibers or SMF-28efibers, and InfmiCor® series multimode fibers.

The interrogator 106 generates the sensing and reference pulses andoutputs the reference pulse after the sensing pulse. The pulses aretransmitted along optical fiber 112 that comprises a first pair of FBGs.The first pair of FBGs comprises first and second FBGs 114 a,b(generally, “FBGs 114”). The first and second FBGs 114 a,b are separatedby a certain segment 116 of the optical fiber 112 (“fiber segment 116”).The optical length of the fiber segment 116 varies in response todynamic strain that the fiber segment 116 experiences.

The light pulses have a wavelength identical or very close to the centerwavelength of the FBGs 114, which is the wavelength of light the FBGs114 are designed to partially reflect; for example, typical FBGs 114 aretuned to reflect light in the 1,000 to 2,000 nm wavelength range. Thesensing and reference pulses are accordingly each partially reflected bythe FBGs 114 a,b and return to the interrogator 106. The delay betweentransmission of the sensing and reference pulses is such that thereference pulse that reflects off the first FBG 114 a (hereinafter the“reflected reference pulse”) arrives at the optical receiver 103simultaneously with the sensing pulse that reflects off the second FBG114 b (hereinafter the “reflected sensing pulse”), which permits opticalinterference to occur.

While FIG. 1A shows only the one pair of FBGs 114 a,b, in alternativeembodiments (not depicted) any number of FBGs 114 may be on the fiber112, and time division multiplexing (TDM) (and, optionally, wavelengthdivision multiplexing (WDM)) may be used to simultaneously obtainmeasurements from them. If two or more pairs of FBGs 114 are used, anyone of the pairs may be tuned to reflect a different center wavelengththan any other of the pairs. Alternatively, a group of multiple FBGs 114may be tuned to reflect a different center wavelength to another groupof multiple FBGs 114, and there may be any number of groups of multipleFBGs extending along the optical fiber 112 with each group of FBGs 114tuned to reflect a different center wavelength. In these exampleembodiments where different pairs or group of FBGs 114 are tuned toreflect different center wavelengths to other pairs or groups of FBGs114, WDM may be used in order to transmit and to receive light from thedifferent pairs or groups of FBGs 114, effectively extending the numberof FBG pairs or groups that can be used in series along the opticalfiber 112 by reducing the effect of optical loss that otherwise wouldhave resulted from light reflecting from the FBGs 114 located on thefiber 112 nearer to the interrogator 106. When different pairs of theFBGs 114 are not tuned to different center wavelengths, TDM issufficient.

The interrogator 106 emits laser light with a wavelength selected to beidentical or sufficiently near the center wavelength of the FBGs 114,and each of the FBGs 114 partially reflects the light back towards theinterrogator 106. The timing of the successively transmitted lightpulses is such that the light pulses reflected by the first and secondFBGs 114 a,b interfere with each other at the interrogator 106, whichrecords the resulting interference signal. The strain that the fibersegment 116 experiences alters the optical path length between the twoFBGs 114 and thus causes a phase difference to arise between the twointerfering pulses. The resultant optical power at the optical receiver103 can be used to determine this phase difference. Consequently, theinterference signal that the interrogator 106 receives varies with thestrain the fiber segment 116 is experiencing, which allows theinterrogator 106 to estimate the strain the fiber segment 116experiences from the received optical power. The interrogator 106digitizes the phase difference (“output signal”) whose magnitude andfrequency vary directly with the magnitude and frequency of the dynamicstrain the fiber segment 116 experiences.

The signal processing device 118 is communicatively coupled to theinterrogator 106 to receive the output signal. The signal processingdevice 118 includes a processor 102 and a non-transitorycomputer-readable medium 104 that are communicatively coupled to eachother. An input device 110 and a display 108 interact with the processor102. The computer-readable medium 104 has stored on it program code tocause the processor 102 to perform any suitable signal processingmethods to the output signal. For example, if the fiber segment 116 islaid adjacent a region of interest that is simultaneously experiencingvibration at a rate under 20 Hz and acoustics at a rate over 20 Hz, thefiber segment 116 will experience similar strain and the output signalwill comprise a superposition of signals representative of thatvibration and those acoustics. The processor 102 may apply to the outputsignal a low pass filter with a cut-off frequency of 20 Hz, to isolatethe vibration portion of the output signal from the acoustics portion ofthe output signal. Analogously, to isolate the acoustics portion of theoutput signal from the vibration portion, the processor 102 may apply ahigh-pass filter with a cut-off frequency of 20 Hz. The processor 102may also apply more complex signal processing methods to the outputsignal; example methods include those described in PCT applicationPCT/CA2012/000018 (publication number WO 2013/102252), the entirety ofwhich is hereby incorporated by reference.

FIG. 1B depicts how the FBGs 114 reflect the light pulse, according toanother embodiment in which the optical fiber 112 comprises a third FBG114 c. In FIG. 1B, the second FBG 114 b is equidistant from each of thefirst and third FBGs 114 a,c when the fiber 112 is not strained. Thelight pulse is propagating along the fiber 112 and encounters threedifferent FBGs 114, with each of the FBGs 114 reflecting a portion 115of the pulse back towards the interrogator 106. In embodimentscomprising three or more FBGs 114, the portions of the sensing andreference pulses not reflected by the first and second FBGs 114 a,b canreflect off the third FBG 114 c and any subsequent FBGs 114, resultingin interferometry that can be used to detect strain along the fiber 112occurring further from the interrogator 106 than the second FBG 114 b.For example, in the embodiment of FIG. 1B, a portion of the sensingpulse not reflected by the first and second FBGs 114 a,b can reflect offthe third FBG 114 c, and a portion of the reference pulse not reflectedby the first FBG 114 a can reflect off the second FBG 114 b, and thesereflected pulses can interfere with each other at the interrogator 106.

Any changes to the optical path length of the fiber segment 116 resultin a corresponding phase difference between the reflected reference andsensing pulses at the interrogator 106. Since the two reflected pulsesare received as one combined interference pulse, the phase differencebetween them is embedded in the combined signal. This phase informationcan be extracted using proper signal processing techniques, such asphase demodulation. The relationship between the optical path of thefiber segment 116 and that phase difference (θ) is as follows:

${\theta = \frac{2\pi\;{nL}}{\lambda}},$where n is the index of refraction of the optical fiber; L is thephysical path length of the fiber segment 116; and λ is the wavelengthof the optical pulses. A change in nL is caused by the fiberexperiencing longitudinal strain induced by energy being transferredinto the fiber. The source of this energy may be, for example, an objectoutside of the fiber experiencing dynamic strain, undergoing vibration,or emitting energy. As used herein, “dynamic strain” refers to strainthat changes over time. Dynamic strain that has a frequency of betweenabout 5 Hz and about 20 Hz is referred to by persons skilled in the artas “vibration”, dynamic strain that has a frequency of greater thanabout 20 Hz is referred to by persons skilled in the art as “acoustics”,and dynamic strain that changes at a rate of <1 Hz, such as at 500 μHz,is referred to as “sub-Hz strain”.

One conventional way of determining Δ nL is by using what is broadlyreferred to as distributed acoustic sensing (“DAS”). DAS involves layingthe fiber 112 through or near a region of interest and then sending acoherent laser pulse along the fiber 112. As shown in FIG. 10, the laserpulse interacts with impurities 113 in the fiber 112, which results inscattered laser light 117 because of Rayleigh scattering. Vibration oracoustics emanating from the region of interest results in a certainlength of the fiber becoming strained, and the optical path change alongthat length varies directly with the magnitude of that strain. Some ofthe scattered laser light 117 is back-scattered along the fiber 112 andis directed towards the optical receiver 103, and depending on theamount of time required for the scattered light 117 to reach thereceiver and the phase of the scattered light 117 as determined at thereceiver, the location and magnitude of the vibration or acoustics canbe estimated with respect to time. DAS relies on interferometry usingthe reflected light to estimate the strain the fiber experiences. Theamount of light that is reflected is relatively low because it is asubset of the scattered light 117. Consequently, and as evidenced bycomparing FIGS. 1B and 1C, Rayleigh scattering transmits less light backtowards the optical receiver 103 than using the FBGs 114.

DAS accordingly uses Rayleigh scattering to estimate the magnitude, withrespect to time, of the strain experienced by the fiber during aninterrogation time window, which is a proxy for the magnitude of thevibration or acoustics emanating from the region of interest. Incontrast, the embodiments described herein measure dynamic strain usinginterferometry resulting from laser light reflected by FBGs 114 that areadded to the fiber 112 and that are designed to reflect significantlymore of the light than is reflected as a result of Rayleigh scattering.This contrasts with an alternative use of FBGs 114 in which the centerwavelengths of the FBGs 114 are monitored to detect any changes that mayresult to it in response to strain. In the depicted embodiments, groupsof the FBGs 114 are located along the fiber 112. A typical FBG can havea reflectivity rating of between 0.1% and 5%. The use of FBG-basedinterferometry to measure dynamic strain offers several advantages overDAS, in terms of optical performance.

Referring now to FIG. 2, there is shown an optical interrogator 300 forperforming interferometry using FBGs, according to one embodiment. Theinterrogator 300 comprises a light source in the form of a laser 302whose output is optically coupled in series to various opticalcomponents; in order from the laser 302 these components are an inputoptical isolator 304 a, an input optical amplifier 308, an outputoptical isolator 304 b, an optical splitter 305, a delay coil 306, anoptical combiner 307, a phase modulator 310, an output optical amplifier314, and a first port of an optical circulator 320. A second port of theoptical circulator 320 is optically coupled to the interrogator's output311. Optically coupled to the interrogator's output 311 is the opticalfiber 112 comprising the FBGs 114 (not shown). A third port of theoptical circulator 320 is optically coupled to a variable opticalattenuator 312 which in turn is coupled to a polarization splitter 313.Polarization splitter 313 splits incoming light according to itspolarization, and in the present embodiment splits and sends incominglight towards receiver circuitry 322, comprising photodiodes 322 a-c. Inthe depicted embodiment, receiver circuitry 322 converts reflected lightpulses into electrical signals, but in alternative embodiments mayconvert the reflected light pulses into a different type of signal, suchas an acoustic signal. The optical circulator 320 directs light pulsesentering its first port out its second port, and directs light pulsesentering its second port out its third port. The effect of this is thatthe sensing and reference pulses are transmitted from the output opticalamplifier 314 to the FBGs 114, while reflected pulses are transmittedfrom the FBGs 114 to the receiver circuitry 322. The optical fiber 112is used to optically couple the components that comprise the laser 302,optical isolator 304 a, optical amplifier 308, optical isolator 304 b,optical splitter 305, delay coil 306, optical combiner 307, phasemodulator 310, optical amplifier 314, optical circulator 320, opticalattenuator 312, polarization splitter 313, and receiver circuitry 322.However, in an alternative embodiment (not depicted), an alternative tothe optical fiber 112 may be used to optically couple the variouscomponents together; for example, free space optical communication maybe used to optically couple the various components together. In anotheralternative embodiment (not depicted), the optical circulator 320 may bereplaced with a package comprising an optical coupler and an opticalisolator.

In FIG. 2, the laser 302 outputs phase coherent light to permit thesensing and reflected pulses to interfere with each other after beingreflected by the FBGs 114. Additionally, while the laser 302 is thelight source in the depicted embodiment, alternative embodiments (notdepicted) may comprise a non-laser coherent light source.

The interrogator 300 also comprises a controller 324 communicativelycoupled to the input optical amplifier 308 and to the phase modulator310 via a digital to analog converter 326 (“DAC 326”) and an analogamplifier 328. The controller 324 is consequently able to control theamplitude of the light pulse generated by the laser 302, as well as thephase modulation of the sensing and reference pulses. The controller 324is configured to perform a method for interrogating the FBGs 114 or forcalibrating the interrogator 300, such as the example methods shown inFIGS. 7 and 13 and described in more detail, below. The controller 324in the depicted embodiment is a field programmable gate array (“FPGA”),which is configured using a hardware description language such as VHDLor Verilog from which a netlist is generated and used to configure theFPGA in the field. The DAC 326 and analog amplifier 328 allow thecontroller 324 to output all digital signals and still be able tocontrol the first optical amplifier 308 and phase modulator 310; in analternative embodiment (not depicted) some or all of the signals thecontroller 324 outputs may be analog signals and the controller 324 mayconsequently be directly communicatively coupled to one or both of theamplifier 308 and phase modulator 310. Alternatively, one or both of theamplifier 308 and phase modulator 310 may be configured to receivedigital input signals, in which case the controller 324 may be directlycommunicatively coupled to one or both of the amplifier 308 and phasemodulator 310 if the controller 324 also outputs at least some digitalsignals. As another alternative (not depicted), one or both of theamplifier 308 and the phase modulator 310 may be configured to receiveanalog signals, the controller 324 may be configured to output at leastsome analog signals, and the controller 324 may be communicativelycoupled to one or both of the amplifier 308 and phase modulator via ananalog to digital converter and, optionally, a digital amplifier.

In this depicted embodiment, the laser 302 generates light centered on1,550 nm and has a narrow line width and a long coherence length. Theinput optical isolator 304 a prevents back reflections fromdestabilizing the laser 302. In this example embodiment the inputoptical amplifier 308 is a semiconductor optical amplifier (“SOA”). Theoutput optical isolator 304 b prevents back reflections fromdestabilizing the first optical amplifier 308. Optical splitter 305 isconfigured to split laser light incident thereon, and send separatepulses along upper and lower optical paths 309 a and 309 b. Delay coil306 is responsible for introducing a delay in the light pulse passingalong lower optical path 309 b relative to the light pulse passing alongupper optical path 309 a. The pair of light pulses (the sensing andreference pulses), delayed relative to each other, are transmitted alonga common optical path once they arrive at optical combiner 307. Thephase modulator 310, which in this example embodiment is a solid statelithium niobate phase modulator, allows the controller 324 to controlphase modulation of one or both of the sensing and reference pulses. Theoutput optical amplifier 314 boosts the power of the sensing andreference pulses for transmission to the FBGs 114; in this exampleembodiment, the output optical amplifier 314 is an erbium doped fiberamplifier (“EDFA”).

Example component manufacturers are Covega™ Technologies for the inputoptical amplifier 308 and the phase modulator 310, Nuphoton™Technologies, Inc. for the output optical amplifier 314, OSI™ LaserDiode Inc. for the receiver circuitry 322, OZ Optics™ Ltd. for thecirculator 320, and Thorlabs™, Inc. for the optical isolators 304 a,b.

Referring now to FIG. 7, there is shown a method 700 for interrogatingthe optical fiber 112, according to another embodiment. As mentionedabove, the method 700 is encoded on to the FPGA that comprises thecontroller 324 as a combination of FPGA elements such as logic blocks.The controller 324 begins performing the method 700 at block 702 andproceeds to block 704 where it generates a light pulse using lightemitted from a light source by modulating the intensity of the light. Togenerate the pulse, the controller 324 controls the input opticalamplifier 308 to modulate the amplitude of the light the laser 302emits. Optical splitter 305 causes the light pulse to be split into apair of light pulses, and subsequently delay coil 306 induces a delay inone pulse relative to the other pulse. These light pulses are thesensing and reference pulses and the light source is the laser 302.

The pulses are amplified by the output optical amplifier 314 and aretransmitted through the optical circulator 320 and to the optical fiber112 and the FBGs 114 (block 706). The pulses are then reflected off theFBGs 114 and return to the interrogator 300 (block 708) where they aredirected via the optical circulator 320 to the receiver circuitry 322,which in the depicted embodiment converts the interference patternresulting from the reflections into an electrical signal. Theinterference patterns resulting from the reflections are then observed,such as at the signal processing software 118, and an operator of theinterrogator 300 can determine whether the optical path length betweenthe FBGs 114 has changed from the interference pattern that results frominterference of the reflections (block 710). For example, the operatorcan make determinations about the nature of the dynamic strainexperienced by the fiber segments 116 between the FBGs 114.

In some alternative embodiments, between blocks 704 and 706 thecontroller 324 phase shifts one of the light pulses relative to theother of the light pulses; that is, in the example embodiment in whichthe sensing and reference pulses are generated, the controller 324causes the phase modulator 310 to phase shift one or both of the sensingand reference pulses. When the phase modulator 310 is a lithium niobatephase modulator, the modulator 310 is able to introduce a phase shift ofup to +/−π to one or both of the sensing and reference pulses; byintroducing a phase shift of as much as +π to one of the pulses and asmuch as −π to the other of the pulses, the controller 324 can introducea phase difference of anywhere from 0 to 2π between the pulses. Incontrast to a conventional piezoelectric fiber stretcher, using alithium niobate phase modulator permits faster phase modulation rates(in the depicted embodiment, the phase modulator 308 can modulate at upto 10 GHz, and alternative and commercially available phase modulators308 can modulate at up to 40 GHz), introduces less noise, and permitsnonlinear and piecewise modulation schemes. For example, any of asinusoidal, sawtooth, triangle, and stepwise function can be used todrive the phase modulator 310, with the light pulses being modulatedaccordingly. In another alternative embodiment, a Barker code may beused for phase modulation.

As alluded to above in respect of FIG. 1A, in some alternativeembodiments (not depicted) the fiber 112 may comprise groups of two ormore of the FBGs 114, with these groups located at different positionsalong the fiber 112 and with the FBGs 114 in any one of these groupstuned to a common center wavelength that is different from the centerwavelength to which the FBGs 114 in the other groups are tuned. Forexample, there may be a first group of three FBGs 114 along the fiber112 extending from 200 m to 600 m from the interrogator 300 and tuned toa first center wavelength, a second group of three FBGs 114 along thefiber 112 extending from 600 m to 1000 m from the interrogator 300 andtuned to a second center wavelength different from the first centerwavelength, and a third group of three FBGs 114 along the fiber 112extending from 1000 m to 1400 m from the interrogator 300 and tuned to athird center wavelength different from the first and second centerwavelengths. In this example, the controller 324 may be configured tocause the interrogator 300 to use TDM to interrogate each of these threedifferent groups of FBGs 114 using pulses of the three differentwavelengths of light launched from the interrogator 300 at differenttimes. For example, a first pulse at the first center wavelength may belaunched for the first group of FBGs 114 at times t1 and t2, a secondpulse at the second center wavelength may be launched for the secondgroup of FBGs 114 at times t3 and t4, and a third pulse at the thirdcenter wavelength may be launched for the third group of FBGs 114 attimes t5 and t6, with t1<t2<t3<t4<t5<t6. In this manner, differentwavelengths of light may be used to interrogate different lengths of thefiber 112. In an alternative embodiment, light pulses having differentwavelengths may be simultaneously launched into the fiber 112; in thisembodiment and applying the terminology of the immediately precedingexample, t1=t3=t5 and t2=t4, with each of t1, t3, and t5>t2, t4, and t6.

Example interference patterns are depicted in FIG. 4. FIG. 4 shows agraph 600 of first through fourth pulses 602 a-d (collectively, “pulses602”) resulting from reflections off of the FBGs 114 of the sensing andreference pulses generated using the interrogator 300 of FIG. 2. Thepulses 602 are measured after the receiver circuitry 322 has convertedthe reflections from an optical to an electrical signal.

The graph 600 is generated by interrogating three of the FBGs 114: thefirst and second FBGs 114 a,b and a third FBG 114 located along theoptical fiber 112 further from the interrogator 300 than the second FBG114 b, with the three FBGs 114 equally spaced from each other. The firstpulse 602 a shows the sensing pulse after it has reflected off of thefirst FBG 114 a; the second pulse 602 b shows the interference resultingfrom the reference pulse after it has reflected off the first FBG 114 aand the sensing pulse after it has reflected off the second FBG 114 b;the third pulse 602 c shows the interference resulting from thereference pulse after it has reflected off the second FBG 114 b and thesensing pulse after it has reflected off the third FBG 114 c; and thefourth pulse 602 d shows the reference pulse after it has reflected offthe third FBG 114.

Any variation in the optical length of the fiber segment 116 between thefirst and second FBGs 114 a,b is reflected in the phase variation of thesecond pulse 602 b. Similarly, any variation in the optical length ofthe fiber segment 116 between the second FBG 114 b and the third FBG 114is reflected in the amplitude variation of the third pulse 602 c. Asdiscussed above in respect of FIGS. 1A-1C, the optical length of thefiber 112 can be changed in response to dynamic strain, of which onetype is strain in the fiber 112 caused by an acoustic signal.

ALTERNATIVE EMBODIMENTS

In addition to the example embodiment of the interrogator 300 shown inFIG. 2, alternative embodiments are possible. Example alternativeembodiments of the interrogator 300 are shown in FIGS. 3 and 4.

FIG. 3 shows an embodiment of the interrogator 300 in which apolarization controller 404 is optically coupled between the phasemodulator 310 and the output optical amplifier 314. In FIG. 3, theoutput optical amplifier 314 and the optical circulator 320 arepolarization maintaining components, and all the fiber 112 between thepolarization controller 404 and the FBGs 114 (including the fibersegment 116) and between the polarization controller 404 and thepolarization splitter 313 is polarization maintaining fiber (“PMF”). Anexample brand of PMF is Panda Fiber™ manufactured by FujikurarM Ltd. Thepolarization controller 404 is actively controlled by, and accordinglycommunicatively coupled to, the controller 324. Regardless of thepolarization of the light entering the polarization controller 404, thepolarization controller 404 converts the polarization of any laser lightexiting the phase modulator 310 into a known polarization, which the PMFmaintains. Both the sensing and reference pulses will consequently enterthe output optical amplifier 314 in the same polarization state, and anychanges in polarization between the output optical amplifier 314 and thereceiver circuitry 322 will be experienced by both pulses except for anypolarization changes occurring in the fiber segments 116 between pairsof the FBGs 114. This helps to keep the polarizations of the sensing andreference pulses aligned, which increases the degree to which the pulsesinterfere and consequently the sensitivity of the interrogator 300. Thepolarization splitter 313 allows either all reflected light or any oneof three polarizations of reflected light, each separated by 120°, topass through to the receiver circuitry 322 while discarding theremaining polarizations. Permitting only one polarization to reach thereceiver circuitry 322 allows the receiver circuitry 322 to discardnoisy data that could reduce the interrogator's 300 sensitivity andaccuracy. The polarization splitter 313 can also be used to permit anycombination of the three polarizations of the reflected light, such asthe sum of any two or all three polarizations of the reflected light, toreach the receiver circuitry 322 if desired.

The polarization controller 404 in FIG. 3 increases component selectionflexibility by permitting selection of a wider range of lasers than whenthe polarization controller 404 is not used. Commercially availablelasers may or may not output light of a fixed polarization; thepolarization controller 404 allows polarization of the laser 302 to beadjusted. Accordingly, the laser 302 need not emit light of a constantand known polarization in order for the interrogator 300 to emit lightof a known polarization to the FBGs 114. Similarly, the polarizationcontroller 404 allows non-PMF to be used between the laser 302 and thepolarization controller 404 and allows the optical components betweenthe laser 302 and the polarization controller 404 to not be polarizationmaintaining while still permitting the interrogator 300 to enjoy atleast some benefits of polarization control. In an alternativeembodiment (such as in FIG. 2), the polarization controller 404 can beomitted from the interrogator 300 of FIG. 3 and the laser 302 can beconfigured to output a known and fixed polarization and be used inconjunction with PMF and polarization maintaining optical components. Inanother alternative embodiment (not depicted), the polarizationcontroller 404 may be located at a different location in theinterrogator 300 than that shown in FIG. 3. For example, the laser 302may be a communication or narrow line width laser purchased inconjunction with the PMF and with the laser polarization aligned to thePMF with the polarization controller 404 located between the laser 302and the phase modulator 310.

In another alternative embodiment (not depicted), the interrogator 300may omit the polarization splitter 313, such as when the optical fiber112 outside of the interrogator 300 (including the fiber 112 comprisingthe FBGs 114) is PMF. In additional alternative embodiments (notdepicted), the interrogator 300 may instead comprise a polarizationseparating component other than the polarization splitter 313. Forexample, the polarization splitter 313 may be replaced with any one ormore of polarization filters of 0°, 45°, and 90°, and open receivers.

In another alternative embodiment (not depicted), the laser 302, inputand output optical isolators 304 a,b, and input optical amplifier 308 ofFIG. 2 or FIG. 3 are replaced with an electroabsorption modulated laser(hereinafter “EML”). The EML comprises an integrated optical isolatorand an absorption region that acts as amplitude modulation circuitry.The controller 324 is communicatively coupled to the EML to permit thecontroller 324 to control amplitude modulation. Using the EML instead ofthe components in FIG. 2 or 3 that it replaces results in component andcost savings and can improve extinction performance relative to using anexternal SOA for amplitude modulation.

In any of the embodiments described herein, some or all of the opticalfiber 112 used to connect the various optical components within theinterrogator 300 may be PMF and the optical components themselves may bepolarization maintaining. As discussed above in respect of FIG. 3,maintaining polarization between the sensing and reference pulses usingPMF can increase the interrogator's 300 sensitivity by using PMFthroughout, and optionally outside, of the interrogator 300. In variantsof the embodiments of FIGS. 2 and 3, for example, PMF may be used tooptically couple only the components between the laser 302 and theinterrogator's output 311, only between the interrogator's output 311and the receiver circuitry 322, only between the laser 302 and the phasemodulator 310, or all optical components within the interrogator 300;and regardless of whether PMF is used to optically couple theinterrogator's 300 internal components together, PMF may be used forsome or all of the optical fiber 112 outside of the interrogator 300 andthat comprises the FBGs 114. Similarly, in the embodiment in which anEML is used, PMF may be used to optically couple only the componentsbetween the EML and the interrogator's output 311, only between theinterrogator's output 311 and the receiver circuitry 322, only betweenthe EML and the phase modulator 310, or all optical components withinthe interrogator 300; and regardless of whether PMF is used to opticallycouple the interrogator's 300 internal components together, PMF may beused for some or all of the optical fiber 112 outside of theinterrogator 300 and that comprises the FBGs 114.

In another alternative embodiment (not depicted), a high-power laser canbe used as a light source in order to eliminate the output opticalamplifier 314. For example, a laser rated at least 100 mW may be used,and the EDFA that acts as the output optical amplifier 314 may beeliminated. This helps to reduce cost and increase SNR. A high-powerlaser can similarly be introduced into the embodiments of FIGS. 2 and 3.

In another alternative embodiment (not depicted), the controller 324 mayimplement dithering in order to reduce the effect of noise resultingfrom leakage crosstalk and spontaneous emissions, for example, andthereby increase SNR. As one example, in the embodiments of FIGS. 2 and3, the input optical amplifier 308, an SOA, is used to generate a lightpulse by modulating the amplitude of the laser light. However, even whenthe amplifier 308 is off (i.e. set to completely extinguish the laserlight), some of the laser light may still be transmitted through theamplifier 308; this light is referred to as “leakage”. The leakage actsas noise and impairs the interrogator's 300 SNR.

The phase modulator 310 may be used to compensate for the leakage bydithering; that is, by phase modulating the leakage at a frequencysubstantially higher than the interrogator's 300 interrogationfrequency. For example, if the interrogator 300 is interrogating theFBGs 114 at a frequency of 4 MHz, the phase modulator 310 may modulatethe leakage at a frequency of 20 MHz while the amplifier 308 is off,with the phase modulation varying the phase of the leakage between 0radians and π radians. When the receiver circuitry 322 receives thereflections from the FBGs 114, the average of the leakage is zero, thusimproving the interrogator's 300 SNR relative to examples wheredithering is not used. In one embodiment, the phase modulator 310modulates the leakage at at least twice the interrogation frequency(i.e., the Nyquist frequency) or at some other even multiple of theinterrogation frequency, which provides a net DC demodulation of thedither. Modulating the leakage at at least 2.5 times the interrogationfrequency provides a potentially useful buffer between the modulationfrequency and the Nyquist frequency. Modulating at higher noise ditherrates, such as at at least ten times the interrogation frequency, insome embodiments permits analog filtering to be applied to the signalthe interrogator 300 receives from the FBGs 114, to reduce costs. Forexample, in one embodiment, modulating the leakage at a rate of at leastone hundred times the interrogation frequency prevents the leakage frombeing able to pass the bandwidth of the receiver circuitry 322, thuspermitting noise filtering without having to add specialized filteringcircuitry over and above what is depicted in FIGS. 2 and 3.

In some embodiments, interrogator 300 includes a GPS receiver (notdepicted) for synchronizing an internal clock of controller 324 with aset of GPS satellites. The controller 324 may be configured tosynchronize interrogation of the optical fibers 112 as a function of asignal received from the GPS receiver. This may be useful in cases whenfiber optic data acquisition needs to be synchronized with externalevents, such as the exact time when a particular seismic event isgenerated.

Referring now to FIG. 5, there is shown an embodiment of theinterrogator 300 designed for multi-fiber optic data acquisition inwhich there are multiple fibers 112, with each of the fibers comprisingdifferent groups of the FBGs 112 that are interrogated using TDM asdescribed above. The interrogator 300 of FIG. 5 is based on theinterrogator 300 of FIG. 2 with the addition of an optical switch 902interposed between the optical circulator 320 and the output of theinterrogator 300, and the presence of switching control circuitry 904that is communicatively coupled to and that controls operation of theoptical switch 902. The switching control circuitry 904 may be, forexample, an application specific integrated circuit, an FPGA, amicroprocessor, a microcontroller, or any other suitable type of analog,digital, or mixed signal circuitry. The control circuitry 904 may bedistinct from the controller 324 as shown in FIG. 5 or alternatively maycomprise part of the controller 324 (not shown). The optical switch 902may be, for example, an EPS0116S switch from EpiPhotonics Corp. of SanJose, Calif. The switching control circuitry 904 is operable to causethe optical switch 902 to select any one of switch outputs A, B, C, andD for outputting the sensing and reference pulses and for receivingreflected pulses. Switch outputs A-D are connected to first throughfourth lengths of fiber 112 a-d (“first through fourth optical fibers112 a-d”). On each of the optical fibers 112 a-d are first through thirdgroupings of FBGs 114 d-f (“first through third FBG groups 114 d-f”).The FBGs 114 comprising the first FBG group 114 d are all tuned toreflect an identical, first wavelength of light; the FBGs 114 comprisingthe second FBG group 114 e are all tuned to reflect an identical, secondwavelength of light that differs from the first wavelength; and the FBGs114 comprising the third FBG group 114 f are all tuned to reflect anidentical, third wavelength of light that differs from the first andsecond wavelengths.

The laser 302 in FIG. 5 (which may be an EML as described above, inwhich case isolator 304 a and amplifier 308 are comprised within laser302) is configured to output light pulses at each of the first, second,and third wavelengths, thus enabling the interrogator 300 of FIG. 5 tobe used for wavelength division multiplexing (“WDM”). The receivercircuitry 322 is similarly photosensitive to the different wavelengthsof light, and consequently is able to receive and output signalscorresponding to the interference patterns generated by the pulses sentat those different wavelengths. In alternative embodiments (notdepicted), different light sources may be used; for example, severaldifferent lasers 302 may be multiplexed together and externallymodulated as opposed to using an absorption region as in the case of anEML.

Referring now to FIG. 6, there is shown an example of pulse timingapplicable to the interrogator 300 of FIG. 5. The switching controlcircuitry 904 instructs the optical switch 902 to transmit along thefirst optical fiber 112 a, and the interrogator 300 then sends a firstpair of pulses 906 a along the first optical fiber 112 a shortly aftertime t0. The first pair of pulses 906 a is transmitted using the firstthrough third wavelengths corresponding to the wavelengths to which thefirst through third FBG groups 114 d-f are tuned to reflect,respectively. The first pair of pulses 906 a (multiplexed using threedifferent wavelengths of light) travels along the first optical fiber112 a, with the first pair of pulses 906 a at the first wavelengthreflecting off the first FBG group 114 d, the first pair of pulses 906 aat the second wavelength reflecting off the second FBG group 114 e, andthe first pair of pulses 906 a at the third wavelength reflecting offthe third FBG group 114 f. The receiver circuitry 322 receives the threeinterference patterns between the end of the first pair of pulses 906 aand time t1; which is shown in FIG. 6. The receiver circuitry 322receives the interference pattern at the first wavelength as reflectedby the first FBG group 114 d, then at the second wavelength as reflectedby the second FBG group 114 e, and then at the third wavelength asreflected by the third FBG group 114 f. The switching control circuitry904 then instructs the optical switch 902 to transmit along the secondoptical fiber 112 b, and the interrogator 300 then analogously transmitsa second pair of pulses 906 b along the second optical fiber 112 bshortly after time t1 and receives interference patterns at the threewavelengths of light between the end of the second pair of pulses 906 band time t2. Similarly, the switching control circuitry 904 theninstructs the optical switch 902 to transmit along the third and fourthoptical fibers 112 c,d, following which the interrogator 300 thenanalogously transmits a third and a fourth pair of pulses 906 c,d alongthe third and fourth optical fibers 112 c,d shortly after times t2 andt3 and receives interference patterns at the three wavelengths of lightbetween the end of the third pair of pulses 906 c and time t3 and thefourth pair of pulses 906 d and time t4, respectively.

In FIG. 6, the different optical fibers 112 a-d may correspond, forexample, to different assets that the interrogator 300 is being used tomonitor. For example, the different optical fibers 112 a-d maycorrespond to different pipelines that the interrogator 300 ismonitoring. For any one of the optical fibers 112 a-d, the different FBGgroups 114 d-f may correspond to different portions of the asset beingmonitored. For example, the different FBG groups 114 d-f may representdifferent lengths of a pipeline. Using multiple wavelengths to monitordifferent portions of a single asset, such as a pipeline, helps toreduce reflection losses and increase signal-to-noise ratio, since fewerof the FBGs 114 are used to reflect any one wavelength of light.

Although the interrogator 300 of FIG. 5 is based on the interrogator 300of FIG. 2, in alternative embodiments (not depicted) the optical switch902 and switching control circuitry 904 may be analogously added to anyone or more of the embodiments of the interrogator 300 shown in FIG. 3.Alternatively, the switching control circuitry 904 and optical switch902 may be added to other, non-depicted embodiments of the interrogator300. Furthermore, although the optical switch 902 in FIG. 5 comprisesthe four outputs A-D, in alternative embodiments (not depicted) theoptical switch 902 may have only two outputs, only three outputs, ormore than four outputs.

In another alternative embodiment, the different optical fibers 112 a-dcan be connected in series by connecting the end of one of the opticalfibers 112 a-d with the beginning of another of the optical fibers 112a-d. The interrogator 300 may then interrogate the different opticalfibers using TDM or WDM, as described above. To reduce reflectionlosses, an optical circulator 320 may be placed in-between each pair ofthe optical fibers 112 a-d, with each of the optical circulators 320redirecting reflections from the FBGs 114 directly to the receivercircuitry 322. For example, an optical circulator placed between thefirst and second optical fibers 112 a,b may redirect reflections fromthe FBG groups 114 d-f in the second optical fiber 112 b to the signalprocessing device 322. Such an embodiment is described in more detailbelow, in connection with FIGS. 8 and 9.

In another alternative embodiment (not depicted), the interrogator 300may comprise the switching control circuitry 904 and the optical switch902 and be configured to transmit along multiple optical fibers, but notuse a WDM-capable light course.

FIGS. 8 and 9 show an embodiment of an architecture that may be used toimplement long-distance interferometry-based acoustic monitoring, inparticular when multiple optical fibers are used. The architecture ofFIGS. 8 and 9 uses multiple interrogators in order to monitor aparticularly long asset. For example, for particularly long pipelines,it may be necessary to employ multiple interrogators in order toaccurately monitor the dynamic strain along optical fiber deployed alongthe entire length of the pipeline.

Turning to FIG. 8, there is shown a light source 850, such as a laser asdescribed above in connection with any of the above-describedembodiments, optically coupled to a light distribution module 852. Lightdistribution module 852 splits incident light among a number of outputs854, each optically coupled to an interrogator 856 a-d (interrogators856). Interrogators 856 are similar to any of the above-describedinterrogators, wherein a light pulse received at the interrogator issplit into a reference pulse and a sensing pulse, delayed relative toeach other, and wherein the split light pulses are subsequentlytransmitted out of the interrogator via an output 858.

Referring now to interrogators 856 a and 856 b, outputs 858 areoptically coupled to sensor loads 860 a and 860 b via sensor load inputs862. Each sensor load comprises a number of optical fiber segments(shown in more detail in FIG. 9), wherein each segment comprises groupsof FGBs as described above. Light reflected from FBGs is returned fromthe sensor load via a sensor load output 864, and is directed to anoptical amplifier 866. The optical amplifiers 866 are configured toincrease the strength of the optical signal received from thereflections off the FBGS, since light reflected off the FBGs will haveundergone a degree of attenuation. The pulses are then returned tointerrogators 856 a,b via interrogator inputs 868, and subsequently toreceiver circuitry 322 (not shown) as described above.

Referring now to interrogators 856 c and 856 d, interrogators 856 c and856 d comprise outgoing optical switches 870 c,d and incoming opticalswitches 872 c,d, unlike interrogators 856 a and 856 b. As explained inmore detail below, this allows interrogators 856 c and 856 d tointerrogate a greater total length of optical fiber, albeit at a reducedduty cycle. Outgoing optical switches 870 c,d are configured toalternately allow transmission of light pulses through outputs 871 c,d,whereas incoming optical switches 872 c,d are configured to alternatelyallow transmission of light pulses through inputs 873 c,d.

In each of interrogators 856 c,d, controller 324 (not depicted) iscommunicatively coupled to outgoing optical switch 870 c,d and incomingoptical switch 872 c,d, and controls operation of outgoing opticalswitch 870 c,d and incoming optical switch 872 c,d such that, when lightis transmitted out of a first output 871 c,d of outgoing optical switch870 c,d, the light is sent to a first sensor load 860 c,d and isreturned to interrogator 856 c,d via a first input 873 c,d of incomingoptical switch 872 c,d. Similarly, each controller 324 controls theoperation of outgoing optical switch 870 c,d and incoming optical switch872 c,d such that, when light is transmitted out of a second output 871c,d of outgoing optical switch 870 c,d, the light is sent to a secondsensor load 860 c′,d′ and is returned to interrogator 856 c,d via asecond input 873 c,d of incoming optical switch 872 c,d. Thus, whereassensor loads 860 a,b are monitored at 100% duty cycle by interrogators856 a,b, sensor loads 860 c,c′,d,d′ are monitored at duty cycles of lessthan 100%, such as 50% each. However, interrogators 856 c,d areconfigured to monitor roughly twice the length of optical fiber as areinterrogators 856 a,b (albeit at the cost of a reduction in thefrequency of the monitoring). Other combinations of duty cycles arepossible, such as 60%/40%, 70%/30%, etc.

Turning to FIG. 9, there is shown in more detail the optical fibersegments that constitute sensor loads 860 a-d′. For the sake of clarity,only the optical fiber segments of sensor load 860 a have been annotatedin FIG. 9, though analogous annotations apply to the remainingcomponents seen in FIG. 9.

An optical splitter 874 is positioned between the interrogator output858 and sensor load input 862, and is configured to split light incidenton an input 876 of splitter 874 and send the split light out multipleoutputs 878 of splitter 874. Each output 878 of splitter 874 isoptically coupled to a respective lead-in optical fiber 880, which doesnot comprise any FBGs. Each lead-in optical fiber 880 is in turnoptically coupled to a first port of a respective optical circulator 882(circulators 882 operate in a similar fashion to circulator 320described above). Physically, the location of circulators 882 correspondto sensor load inputs 862 seen in FIG. 8. The second port of eachcirculator 882 is optically coupled to an optical fiber segment 884.Each optical fiber segment 884 comprises one or more groups of FBGs (notshown), as described above. Each third port of optical circulators 882is optically coupled to a respective return optical fiber 886 (whichdoes not comprise FBGs) leading to a filter and balance unit (FBU) 888.Light output from FBUs 888 is directed into an optical combiner 890which directs light incident on its multiple outputs 892 out via asingle output 894. The light output from combiner 894 is directed tointerrogator input 868, and subsequently to receiver circuitry 322 asdescribed above.

Thus, the reference and sensing pulses emitted from interrogators 856are directed to optical splitters 874, whereupon the pulses are splitinto a number of pairs of reference and sensing pulses, each pair ofpulses being directed along a lead-in optical fiber 880 towards acirculator 882. Upon arriving at circulator 882, each pair of pulses isdirected along an optical fiber segment 884. Reflections of each pulsepair off FBGs located on the optical fiber segments 884 are returned tocirculators 882 and redirected along return optical fibers 886 towardsFBUs 888. Once output from FBUs 888, each pulse pair enters combiner 890and exits combiner 890 via output 894. The pulse pairs are then returnedto interrogators 856 and directed to receiver circuitry 322 forprocessing, as described above.

FBUs 888 serve a two-fold purpose. They serve to filter out ASE(amplified spontaneous emission) which may have been generated by someof the optical components inside or outside the interrogator 856.Furthermore, they serve to balance the optical power received at theinterrogator 856. Different pulse pairs will have undergone differentattenuations as they are reflected from different groups of FBGs indifferent optical fiber segments 884. Thus, FBUs 888 may equalize theintensity of each returning pulse in order to make maximal use of thereceiver's dynamic range.

Note that in the embodiments of FIGS. 8 and 9, circulators 882effectively replace the circulator 320 seen in the embodiments of FIGS.2, 3 and 5.

Different pulse pairs may be distinguished by using WDM as describedabove, in which case FBGs along a given optical fiber segment 884 aretuned to a different center wavelength than FBGs disposed along anotheroptical fiber segment 884. Furthermore, TDM may be used to de-referencethe pulse pairs emitted from the interrogators 856, by distinguishingthe pulse pairs based on their time of flight. For example, TDM may beused while also using FBGs tuned to different center wavelengths. Thus,multiple light sources and FBGs of different wavelengths may be used tocover larger distances, while still using TDM to de-reference the returnpulses based on their time of flight.

Vibration and Thermal Compensation

In some embodiments, interrogator 300 comprises a 3D accelerometer (notshown). The accelerometer may be positioned adjacent controller 324, forexample on a PCB on which is located controller 324. Vibrationsexperienced by interrogator 300 may couple to various components ofinterrogator 300, such as delay coil 306 and/or laser 302, and appear ascommon mode signals on the phase data obtained during interrogation ofoptical fiber 112. Controller 324 may therefore carry out a method forcompensating for vibrations experienced by interrogator 300.

In one embodiment, the method comprises determining a correlationbetween acceleration data obtained from the accelerometer, and the phasedata obtained from interrogation of the optical fiber 112. Theacceleration data and the phase data may be obtained during a diagnosticor training phase; in other words at a time during which interrogator300 is not being actively used to optical fiber 112. There are variousways known to one of skill in the art regarding how a correlationbetween two variables may be determined. For example, controller 324 maydetermine the cross-correlation between the acceleration data and phasedata. Alternatively or in addition, controller 324 may apply one or moreadaptive filters to the acceleration data and phase data. During amonitoring phase, i.e. at a time when interrogator 30 is being used tointerrogate optical fiber 112, controller 324 adjusts the phase dataobtained as a function of the correlation, in order to compensate forthe vibrations experienced by interrogator 300. Thus, the effects ofvibrations of the interrogator 300 may be largely eliminated. A typicalaccelerometer may have three-dimensional sensitivity and a sensing rangeof +/−2 g.

In addition, or alternatively, interrogator 300 may comprise atemperature sensor (not depicted) for measuring a temperature inside theinterrogator 300, for example in close proximity to compensator delaycoil 306. Changes in temperature of delay coil 306 directly affect theoptical path length of the delay coil 306. Thus, in an analogous fashionto the method described above for compensation for vibrations of theinterrogator 300, the effects of temperature on the delay coil 306 maybe compensated by determining the correlation between phase data andtemperature data obtained from the temperature sensor.

It is envisaged that the effect of other parameters on the phase datamay also be accounted for, by using appropriate sensors in order measurethe effect such parameters. By determining the correlation betweenmeasurements taken from such sensors, and the phase data obtained duringsuch measurements, phase data obtained during an interrogation of theoptical fiber 112 may be adjusted as a function of the correlation.

Firmware Data Flow

FIGS. 12A and 12B show a data flow of the control circuitry of theinterrogator, according to one embodiment. This data flow may be carriedout by control circuitry of any of the above-described interrogatorembodiments. Control circuitry includes controller 324 and one or moreprocessors (not depicted), such as a CPU, that are communicative withcontroller 324.

The process begins by entering a number of component initializationsteps before data collection commences. At step 1200, the controlcircuitry is initiated. At step 1202, the interrogator determineswhether the initialization of the control circuitry was successful. Ifnot, then at step 1204 one or more LEDs are toggled to indicate theinitialization failure to the user. If the initialization is successful,then at step 1206 one or more LEDs are toggled to indicate theinitialization success to the user. At step 1208, the interrogatorinitiates the network communicatively coupling the interrogator to thesignal processing device 118, and initiates DMA. At step 1210, theinterrogator starts a lightweight IP network. The lightweight IP networkprovides the lightweight implementation of the TCP/IP networking stack.At step 1212, if two CPUs are being used, then a CPU of the controlcircuitry is initiated. Step 1212 may be omitted if only one CPU isbeing used. At step 1214, parameter configuration is powered on. At step1216, input amplifier 308 is tested. At step 1218, RAMP values for thephase modulator 310 are loaded. The RAMP values define the step sizeswhich the phase modulator 310 will use to modulate the phases of thesensing and reference pulses.

Turning to FIG. 12B, at step 1220, the process enters a while loop inwhich at step 1222 a user command received over the network isprocessed. At step 1224, the interrogator determines whether the usercommand is for a data collection mode. If not, then the process moves tostep 1226 where the interrogator enters an idle state. If so, then atstep 1228 the control circuitry determines which collection mode hasbeen requested by the user. If the user has requested ADC data, then atstep 1230 the FIFO is cleared (see below), and DMA is reset. At step1232, ADC data collection is enabled. ADC mode is an alternative tophase mode, and can be selected by the user from the user interface ofthe signal processing device 118. ADC mode is a mode in which theinterrogator transmits the ADC output counts to the signal processingdevice 118, instead of extracted phase data. This can be useful in theinitial setup and tuning of the system, for example by allowing the userto specify the sample positions of the received pulses, or to see if anyerroneous reflections are present in the system.

The number of optical signals reflected off the FBGs, and the arrivaltiming of these reflections, depend on the particular sensor array beingdeployed. The interrogator has no prior knowledge of this and thereforethe expected number of received pulses and their timing sequence must bespecified before phase data acquisition can begin. The interrogator istypically entered into ADC mode (steps 1230 and steps 1232) during aninitial setup or calibration phase. In this mode, the interrogator maydetermine the particular points in time at which to sample the opticalsignals returned to the interrogator, using analogue to digitalconverters (ADCs) comprised in receiver circuitry 322. Turning to FIGS.12C and 12D, there is shown one such method of automatically searchingfor sampling positions, in ADC mode.

At step 1250, one acoustic frame of data is sampled by the ADCs. In thepresent embodiment, three ADCs are used (one for each component of lightsplit from polarization splitter 313), although in other embodimentsmore or fewer ADCs may be used. In the present embodiment, the ADCsampling rate is 100 MHz, although other rates may be used. At step1252, a check is carried out to determine whether data from all threeADCs have been analyzed. If yes, then the process moves to step 1264. Ifnot, then the process moves to step 1254 where the data sampled from oneof the ADC channels is extracted. At step 1256, an edge detectionalgorithm (of which various ones are known in the art) is applied to theextracted ADC data. At step 1258, the positions of the detected edgesare obtained. The edges correspond to the points in time when areflected pulse (whether interfering with another pulse or not) is seenat receiver circuitry 322 to be rising or falling. For example, withreference to FIG. 4, there can be seen eight edges 606 corresponding tofour reflections that would be detected by the edge detection algorithm.

At step 1260, a check is carried out to determine whether the number ofdetected edges corresponds to the number of expected reflections.Generally, if there are n FBGs in the sensor array, there are n+1reflected pulses, leading to 2(n+1) edges, half of which are risingedges of the reflected pulses and the other half corresponding to thefalling edges of those pulses. At step 1262, the position of each edgefor the particular ADC channel is recorded. At steps 1264 and 1266, thepositions of the edges detected in each of the ADC channels arecompared. If they are the same, then the edge positions are output tocontroller 324 (step 1270) which stores them in memory (see below). Ifthe positions do not match, then those edge positions with the greatestcorresponding optical power are selected for storage in memory (step1268). As will be described below in more detail, the stored edgepositions are used when interrogator is interrogating optical fiber, sothat the interrogator is able to sample the reflected pulses at theright point in time, for extracting meaningful phase data.

Note that in ADC mode, the phase modulator 310 is typically deactivatedso that the return pulses received at receiver circuitry 322 aresubstantially flat-topped, and therefore are more easily analyzed fortheir edges.

Furthermore, the above-described method is merely exemplary in nature,and in other embodiments one or more steps may be omitted and/orreordered.

Returning to FIG. 12B, if the user has requested phase data, then atstep 1234 the FIFO is cleared, and DMA is reset. At step 1236 phase datacollection is enabled, e.g. by interrogating the optical fiber, asdescribed above. If the user has requested Lissajous data, then at step1238 the FIFO is cleared, and DMA is reset. At step 1240 Lissajous datacollection is enabled (see below). A user may request both phase dataand Lissajous data at the same time.

The process then moves to step 1242 where one or more LEDs are toggledto indicate the data transfer status. At step 1244, DMA data is read andtransferred to the signal processing apparatus 118 as described below inconnection with FIG. 11. The process moves to step 1246 where anyfurther user inputs are processed in which case the process repeats thewhile loop by returning to step 1220. At step 1248 the process ends.

The above-described method is merely exemplary in nature, and in otherembodiments one or more steps may be omitted and/or reordered.

In embodiments, controller 324 is configured to digitize the electricalsignals output by receiver circuitry 322 into data packets, using ADCsas described above and then by mathematically extracting the phase datafrom the digitized interference patterns. As can be seen in FIG. 10,each data packet 1000 includes a preamble 1002 and a payload 1004. Thepreamble 1002 comprises a key 1006 and a frame number 1008, and thepayload 1004 comprises 2 bits of ADC out-of-range (OR) indicators 1010,2 bits of PDR channel selection information 1012, 8 bits indicating theparticular channel number 1014 (i.e. the particular group of FBGs fromwhich the pulses were reflected), and 20 bits of phase data 1016(comprising information relating to the interference between thereference and sensing pulses). ADC OR indicators 1010 indicate when theADCs, comprised in receiver circuitry 322 are receiving input valueswhich exceed their full-scale input range. The 2 bits of PDR channelselection information 1012 inform the user about the particular PDR maskwhich was used to extract the phase data. The PDR masks are used toobserve the different polarized components of the light emitted frompolarization splitter 313. The 2 bits can contain one of 4 possiblevalues (0, 1, 2, 3). This information can be logged for debug purposesby the signal processing device 118. For example, if the operatornotices that at a particular point in time the phase data exhibitsunexpected characteristics, he or she may check the debug file to seewhich PDR mask was being used at that given point in time. The framenumbers of consecutively assembled data packets are incremented by one.In other embodiments, the data packets may take other forms.

The process of assembling the data packets 1000 and transferring thedata packets to signal processing device 118 is shown in FIG. 11. Atstep 1020, the digitized data is transferred to a FIFO (first in, firstout) queue. At step 1022, the data contained in the FIFO is transferredto a block memory (which may also be referred to as block RAM) locatedinside controller 324. At step 1024, the data undergoes samplingselection control. The data in the block memory is originally sampledfrom the ADCs. This continuously acquired block of data comprises therecords of the optical signal from the optical fiber. However, onlythose data segments related to the two optical pulses across theselected optical fiber segments will be useful and will be extracted.Thus, only those ADC samples which correspond to pulses returning fromFBGs are required. A sampling selection control module therefore selectsthe appropriate data points based on pre-specified sampling positionsand outputs five data points for demodulation purposes per channel tothe next module for further processing. In other embodiments, more orfewer data points may be used for phase extraction. The sample positionsare pre-specified using the stored edge positions determined using, forexample, the method of FIGS. 12C and 12D.

At step 1026, the data undergoes PDR data processing. The PDR dataprocessing completes the following tasks: measurement processing, PDRmask selection logic, phase correction and data packaging. Themeasurement processing comprises a demodulation process and outputs thephase through computations based on the obtained five data points andthe mask switching scheme. The PDR mask selection logic comprisesdifferent modes such as fixed mode, one-time mode, and normal mode. Thefixed mode is specified through configuration and, in this mode, thesame mask (specified by the user) will be used at all times. Theone-time mode chooses the maximum intensity mask at the beginning andwill lock onto that mask, without switching to other masks. The normalmode will start with the maximum intensity mask and switch to anothermask whose received power exceeds the current mask's power by a certainthreshold level (pre-specified by the user). After the phase iscalculated, it is further compensated through the phase correctionprocedure. The corrected phase, together with other information such asADC status, mask and channel, are packaged into a 32-bit data packet asdescribed above in connection with FIG. 10. This data sequence isfurther appended with a key 1006 and frame number 1008.

The data packet is then transferred to AXI FIFO (step 1028). AXI FIFOprovides buffering based on FIFO and follows the AXI4 interfaceprotocol. Subsequently, the data packet is transferred to AXI DMA (step1030). AXI DMA provides direct memory access between the memory (DRAM)and the AXI FIFO. At step 1032, the data packet is transferred to an A9processor, and subsequently to DRAM (step 1034). Lastly, the data packetis transferred from interrogator 300 to signal processing device 118,using a communication method with a relatively large data throughput,such as a gigabit Ethernet cable, capable of a throughput of at least 1Gb/s.

The above-described method is merely exemplary in nature, and in otherembodiments one or more steps may be omitted and/or reordered.

Error Checking

In embodiments, once a data packet 1000 is received at signal processingdevice 118, signal processing device 118 is configured to carry out amethod for checking an integrity of the data packet 1000. In particular,signal processing device 118 determines whether the data packet 1000meets a data error condition. In one embodiment, determining whether thedata packet 1000 meets the data error condition comprises determining ifthe difference between any two consecutive keys' locations is equal. Inanother embodiment, determining whether the data packet 1000 meets thedata error condition comprises determining if the frame numbers of anytwo consecutive data packets 1000 meet a predetermined requirement. Inone embodiment, the predetermined requirement comprises the second framenumber (e.g. the frame number of the later-received data packet) havinga value which is one greater than the first frame number (e.g. the framenumber of the earlier-received data packet). In another embodiment, thepredetermined requirement comprises the keys of any two consecutive datapackets to be separated by a preset number of bits. If no data errorcondition is met, then signal processing device 118 determines that thedata packet is error-free and extracts the phase data 1016 from thepayload 1004. The phase data 1016 may be converted to another formatsuch as Matlab® for further processing.

If signal processing device 118 determines that the data error conditionhas been met, then signal processing device 118 marks the data packet1000 as an erroneous data packet, in order to avoid further errors inthe subsequent error checking. For example, when error checking the nextdata packet, the signal processing device 118 will ignore any previousdata packets that have been found to fail the error check.

Lissaious Data

When processing the interference pattern of linearly modulated pulses,the magnitudes of the five ADC sample points on the receivedinterference pattern can be input into a mathematical formula to extractquadrature measures whose magnitudes are related to the received opticalpower, as well as the sine and cosine of the phase angle embedded in theinterference pattern. The phase angle is then the arctangent of thequadrature measures.

Lissajous data comprises the sine and cosine pairs for each sample, andmay be transmitted by interrogator 300 to signal processing device 118.The Lissajous data contains these sine and cosine values. Signalprocessing device 118 can use these sine and cosine values to calculatethe quality of the received optical data. Mathematically, if the sineand cosine values are plotted for a number of received Lissajoussamples, the results should fall on the outline of a perfect circle.However, in practice other shapes can be obtained, such as ellipses,indicating that the sine and cosine calculations are not perfect. Thismay indicate that the optical quality of the data is in question andthat the calculations are not fully reliable.

Controller 324 is configured to interleave the Lissajous data (for all 3PDR masks) with the phase data and send it to signal processing device118 in real time. The Lissajous data may be helpful in extracting anoptical figure of merit from the data. A typical figure of merit is ofthe form (mean(R))/(standard deviation (R)), where R is the sum of thesquares of the sine and cosine terms used to calculate the phase angle.

Calibration

Referring now to FIG. 13, there is shown a method 800 for calibratingthe interrogator 300, according to another embodiment. The method 800may be encoded onto the FPGA that comprises the controller 324 as acombination of FPGA elements such as logic blocks. The method 800 isdescribed below in conjunction with the interrogator 300 of FIG. 2,although it may also be performed using other embodiments of theinterrogator 300, such as the embodiment of FIG. 3.

When performing the method 800, the controller 324 begins at block 802and proceeds to block 804 where it transmits a calibration pulse to theFBGs 114. This calibration pulse may or may not be phase adjusted usingthe phase modulator 310. The calibration pulse is reflected off each ofthe FBGs 114 and the reflected pulses return to the interrogator 300 andare received by the receiver circuitry 322 (block 806). The pulse thatreflects off the first FBG 114 a returns to the receiver circuitry 322first and has the highest amplitude of the reflected pulses; the pulsethat reflects off the second FBG 114 b is the second reflected pulse toarrive at the receiver circuitry 322 and has the second highestamplitude, and this pattern continues for the reflections off theremaining FBGs 114. The controller 324 at block 808 determines thetiming between the sensing and reference pulses based on differences inwhen the reflections of the calibration pulse are received at thereceiver circuitry 322. Determining the arrival times of the calibrationpulses may also be helpful in understanding the spatial separation ofthe FBGs in the sensor array, especially if the locations of some FBGshave changed for some reason.

The timing between the sensing and reference pulses can be controlled bythe delay induced by the delay coil 306 or other optical delayers. Insome embodiments (not shown), there may be multiple optical delay coilsand an associated optical switch for switching transmission of lightbetween the delay coils. Each delay coil may be configured to induce adifferent delay to light entering the delay coil. Calibration of theinterrogator may comprise selecting a particular delay coil, using theoptical switch, based on differences in when the reflections of thecalibration pulse are received at the receiver circuitry 322. In anotherembodiment (not shown), a delay-on-chip circuit may act as the opticaldelayer and may be configured to induce a user-selectable, variabledelay. In such a case, calibrating the interrogator may compriseconfiguring the delay-on-chip circuit to induce a particular delay,based on differences in when the reflections of the calibration pulseare received at the receiver circuitry 322.

In one embodiment, second order reflections from the FBGs 114 (i.e.,reflections of reflections) are mitigated using digital signalprocessing techniques such as infinite impulse response or finiteimpulse response filters, or through suitable modulation such as withBarker codes.

A calibration pulse can also be used to level power between multiplelasers when wavelength division multiplexing is being used, and toadjust gain of the various amplifiers 308,314 in the interrogator 300.

Calibration using a calibration pulse can be done at initial setup ofthe interrogator 300 or periodically while using the interrogator 300 tointerrogate the optical fiber 112. The interrogator 300 can berecalibrated as desired; for example, depending on factors such asthermal changes, mechanical changes (e.g. geotechnical shifts), and longterm fiber stretching.

As discussed above, while the phase modulator 310 in the aboveembodiments is a lithium niobate phase modulator, in alternativeembodiments (not depicted) different types of phase modulators may beused. Example alternative phase modulators are gallium arsenide phasemodulators and indium phosphide phase modulators. The phase modulator310 may or may not be a Mach Zehnder-type modulator.

Aside from an FPGA, the controller 324 used in the foregoing embodimentsmay be, for example, a processor, a microprocessor, microcontroller,programmable logic controller, or an application-specific integratedcircuit. For example, in one alternative embodiment, the controller 324collectively comprises a processor communicatively coupled to anon-transitory computer readable medium that has encoded on it programcode to cause the processor to perform the example methods describedherein. Examples of computer readable media are non-transitory andinclude disc-based media such as CD-ROMs and DVDs, magnetic media suchas hard drives and other forms of magnetic disk storage,semiconductor-based media such as flash media, random access memory, andread only memory.

It is contemplated that any part of any aspect or embodiment discussedin this specification can be implemented or combined with any part ofany other aspect or embodiment discussed in this specification.

For the sake of convenience, the example embodiments above are describedas various interconnected functional blocks. This is not necessary,however, and there may be cases where these functional blocks areequivalently aggregated into a single logic device, program or operationwith unclear boundaries. In any event, the functional blocks can beimplemented by themselves, or in combination with other pieces ofhardware or software.

While particular embodiments have been described in the foregoing, it isto be understood that other embodiments are possible and are intended tobe included herein. It will be clear to any person skilled in the artthat modifications of and adjustments to the foregoing embodiments, notshown, are possible.

The invention claimed is:
 1. An optical fiber interrogator for interrogating optical fiber comprising fiber Bragg gratings (“FBGs”), the interrogator comprising: (a) a light source operable to emit phase coherent light; (b) amplitude modulation circuitry optically coupled to the light source and operable to generate one or more light pulses from the light; (c) an optical splitter optically coupled to the amplitude modulation circuitry and being configured to split a light pulse received from the amplitude modulation circuitry into a pair of light pulses; (d) an optical delayer optically coupled to the optical splitter and configured to introduce a delay to one light pulse of the pair of light pulses relative to the other light pulse of the pair of light pulses; (e) control circuitry comprising a controller, communicatively coupled to the amplitude modulation circuitry, and configured to perform a method for interrogating the optical fiber comprising generating a light pulse by using the amplitude modulation circuitry to modulate light emitted by the light source, wherein the generated light pulse is split into a pair of light pulses by the optical splitter, and wherein one of the light pulses is delayed relative to the other light pulse by the optical delayer; and (f) a phase modulator optically coupled to the amplitude modulation circuitry and operable to introduce a phase shift to at least one of the light pulses, wherein the phase modulator is a solid state phase modulator, and wherein the method further comprises phase shifting each of the light pulses by using the phase modulator.
 2. The interrogator of claim 1, further comprising polarization maintaining fiber between the light source and the phase modulator such that a polarization of the light emitted by the light source is maintained from the light source to the phase modulator.
 3. The interrogator of claim 1, further comprising: (a) an output optical amplifier optically coupled to the phase modulator; (b) receiver circuitry; and (c) an optical circulator comprising first, second, and third ports, wherein the first port is optically coupled to the output optical amplifier, the second port is optically coupled to an output of the interrogator for respectively sending and receiving the pair of light pulses to and from the optical fiber, and the third port is optically coupled to the receiver circuitry for processing signals received from the optical fiber.
 4. The interrogator of claim 3, further comprising an optical attenuator optically coupled between the third port of the optical circulator and the receiver circuitry, for attenuating an intensity of light input to the optical attenuator.
 5. The interrogator of claim 1, further comprising an optical combiner optically coupled to the optical delayer and the optical splitter, the optical combiner being configured to receive the pair of lights pulses via respective inputs of the optical combiner, and transmit the pair of light pulses via a common output of the optical combiner.
 6. The interrogator of claim 1, wherein the interrogator is configured to interrogate multiple optical fibers, the interrogator further comprising an outgoing optical switch optically coupled to the light source and comprising at least two switch outputs, the outgoing optical switch being operable to switch transmission of light between each of the at least two switch outputs, and wherein the control circuitry is further communicatively coupled to the outgoing optical switch and configured to perform the method for interrogating each of the multiple optical fibers, comprising: generating a light pulse by using the amplitude modulation circuitry to modulate light emitted by the light source, wherein the generated light pulse is split into a pair of light pulses by the optical splitter, and wherein one of the light pulses is delayed relative to the other light pulse by the optical delayer; and controlling the outgoing optical switch to switch transmission between the at least two switch outputs.
 7. The interrogator of claim 6, further comprising an incoming optical switch optically coupled to the receiver circuitry and comprising at least two switch inputs, the incoming optical switch being operable to switch transmission of light between each of the at least two switch inputs.
 8. The interrogator of claim 7, wherein the control circuitry is further communicatively coupled to the incoming optical switch and further configured to perform the method for interrogating each of the multiple optical fibers, comprising controlling the incoming optical switch to switch transmission between the at least two switch inputs.
 9. The interrogator of claim 8, wherein the control circuitry is further configured to perform the method for interrogating each of the multiple optical fibers, comprising controlling the outgoing optical switch to switch transmission between the at least two switch outputs, and controlling the incoming optical switch to switch transmission between the at least two switch inputs, such that the multiple optical fibers are interrogated at controllable duty cycles.
 10. The interrogator of claim 1, wherein the controller is configured to determine phase data from interference of reflections of one of the light pulses off the FBGs with reflections of the other light pulse off the FBGs, the interrogator further comprising an accelerometer for obtaining acceleration data related to vibrations of the interrogator, wherein the controller is communicatively coupled to the accelerometer and configured to carry out a method comprising: receiving the acceleration data; determining a correlation between the acceleration data and the phase data; and adjusting the phase data as a function of the correlation so as to compensate for the vibrations.
 11. The interrogator of claim 10, wherein the vibrations of the interrogator comprise vibrations of one or more of: the delay coil, the light source, and the phase modulator.
 12. The interrogator of claim 1, wherein the controller is configured to determine phase data from interference of reflections of one of the light pulses off the FBGs with reflections of the other light pulse off the FBGs, the interrogator further comprising a temperature sensor for obtaining temperature data related to a temperature of the interrogator, wherein the controller is communicatively coupled to the temperature sensor and configured to carry out a method comprising: receiving the temperature data; determining a correlation between the temperature data and the phase data; and adjusting the phase data as a function of the correlation so as to compensate for the temperature.
 13. The interrogator of claim 1, further comprising a GPS receiver, wherein the controller is configured to synchronize interrogation of the optical fiber as a function of a signal received from the GPS receiver.
 14. The interrogator of claim 1, wherein the controller is further configured to determine Lissajous data from interference of reflections of one of the light pulses off the FBGs with reflections of the other light pulse off the FBGs.
 15. The interrogator of claim 14, wherein the controller is further configured to determine Lissajous data from the interference of reflections of one of the light pulses off the FBGs with reflections of the other light pulse off the FBGs, during interrogation of the optical fiber.
 16. The optical fiber interrogator of claim 1, wherein the solid state phase modulator is selected from a group consisting of a lithium niobate phase modulator, a gallium arsenide phase modulator, and an indium phosphide phase modulator.
 17. The optical fiber interrogator of claim 1, wherein phase shifting each of the light pulses comprises introducing a phase shift of up to π radians to one of the light pulses, and introducing a phase shift of up to −π0 radians to the other of the light pulses.
 18. A system for interrogating optical fiber comprising fiber Bragg gratings (“FBGs”), the system comprising: (a) an optical fiber interrogator comprising: (i) a light source operable to emit phase coherent light; (ii) amplitude modulation circuitry optically coupled to the light source and operable to generate one or more light pulses from the light; (iii) an optical splitter optically coupled to the amplitude modulation circuitry and being configured to split a light pulse received from the amplitude modulation circuitry into a pair of light pulses; (iv) an optical delayer optically coupled to the optical splitter and configured to introduce a delay to one light pulse of the pair of light pulses relative to the other light pulse of the pair of light pulses; (v) control circuitry comprising a controller, communicatively coupled to the amplitude modulation circuitry, and configured to perform a method for interrogating the optical fiber comprising generating a light pulse by using the amplitude modulation circuitry to modulate light emitted by the light source, wherein the generated light pulse is split into a pair of light pulses by the optical splitter, and wherein one of the light pulses is delayed relative to the other light pulse by the optical delayer; and (vi) a phase modulator optically coupled to the amplitude modulation circuitry and operable to introduce a phase shift to at least one of the light pulses, wherein the phase modulator is a solid state phase modulator, and wherein the method further comprises phase shifting each of the light pulses by using the phase modulator; (b) one or more optical fiber segments optically coupled to the interrogator; and (c) an outgoing optical splitter and an incoming optical combiner, the outgoing optical splitter being optically coupled to the light source and being configured to split light received at the outgoing optical splitter and transmit the split light out each of multiple outputs of the outgoing optical splitter, and wherein the incoming optical combiner is optically coupled to the receiver circuitry and is configured to combine light received at each of multiple inputs of the incoming optical combiner and transmit the combined light to the receiver circuitry.
 19. The system of claim 18, further comprising one or more filter and balance units optically coupled to one or more of the multiple inputs of the incoming optical combiner.
 20. The system of claim 18, further comprising one or more optical circulators optically coupled to each of the one or more optical fiber segments, wherein, for each optical fiber segment, light sent from the interrogator to the optical fiber segment passes through the optical circulator, is reflected off the FBGs comprised in the optical fiber segment, and is redirected by the circulator to the receiver circuitry, and wherein the system further comprises one or more lead-in optical fiber segments optically coupling the interrogator to each of the one or more optical circulators, and one or more return optical fiber segments optically coupling each of the one or more optical circulators to the receiver circuitry.
 21. The system of claim 20, wherein the one or more lead-in optical fiber segments are optically coupled to the multiple outputs of the outgoing optical splitter, wherein the one or more return optical fiber segments are optically coupled to the multiple inputs of the incoming optical combiner, and wherein the one or more return optical fiber segments are optically coupled to one or more filter and balance units optically coupled to one or more of the multiple inputs of the incoming optical combiner.
 22. The system of claim 20, wherein the one or more lead-in optical fiber segments and the one or more return optical fiber segments do not comprise FBGs.
 23. A method for interrogating optical fiber comprising fiber Bragg gratings (“FBGs”), using an optical fiber interrogator, the method comprising: (a) generating an initial light pulse from phase coherent light emitted from a light source, wherein the initial light pulse is generated by modulating the intensity of the light; (b) splitting the initial light pulse into a pair of light pulses; (c) causing one of the light pulses to be delayed relative to the other of the light pulses; (d) phase shifting each of the light pulses by using a solid state phase modulator comprised in the optical fiber interrogator; (e) transmitting the light pulses along the optical fiber; (f) receiving reflections of the light pulses off the FBGs; and (g) determining whether an optical path length between the FBGs has changed from an interference pattern resulting from the reflections of the light pulses.
 24. The method of claim 23, further comprising determining phase data from interference of reflections of one of the light pulses off the FBGs with reflections of the other light pulse off the FBGs, and assembling the phase data into data packets, each data packet comprising a key, a frame identifier and a payload comprising at least a portion of the phase data.
 25. The method of claim 24, further comprising determining whether any of the data packets meet a data error condition and, if so, adding an indication to the data packet that the data packet contains erroneous data.
 26. The method of claim 25, wherein determining the data error condition comprises determining whether the frame identifiers of consecutively assembled data packets do not meet a predetermined requirement; or determining whether the keys of consecutively assembled data packets do not meet a predetermined requirement.
 27. The method of claim 26, wherein the predetermined requirement comprises the frame number of an earlier assembled data packet being one less than the frame number of the later, consecutively assembled data packet, or wherein the predetermined requirement comprises the key of one of the consecutively assembled data packets being separated from the key of the other of the consecutively assembled data packets by a preset number of bits.
 28. A non-transitory computer readable medium having stored thereon program code to cause a processor to perform a method for interrogating optical fiber comprising fiber Bragg gratings (“FBGs”), using an optical fiber interrogator, the method comprising: (a) generating an initial light pulse from phase coherent light emitted from a light source, wherein the initial light pulse is generated by modulating the intensity of the light; (b) splitting the initial light pulse into a pair of light pulses; (c) causing one of the light pulses to be delayed relative to the other of the light pulses; (d) phase shifting each of the light pulses by using a solid state phase modulator comprised in the optical fiber interrogator; (e) transmitting the light pulses along the optical fiber; (f) receiving reflections of the light pulses off the FBGs; and (g) determining whether an optical path length between the FBGs has changed from an interference pattern resulting from the reflections of the light pulses. 